Scanning probe-based lithography method

ABSTRACT

A resist medium in which features are lithographically produced by scanning a surface of the medium with an AFM probe positioned in contact therewith. The resist medium comprises a substrate; and a polymer resist layer within which features are produced by mechanical action of the probe. The polymer contains thermally reversible crosslinkages.

FIELD OF THE INVENTION

The present invention relates generally to a scanning probe-basedlithography (hereinafter SPL), and in particular to an SPL method inwhich patterning of a resist medium is produced by Atomic ForceMicroscope (hereinafter AFM) probe-surface contact. In particular itrelates to the “writing” of a pattern of lines and similar features in aresist. More particularly, the invention relates to a polymer resist foruse in a scanning probe-based lithography method.

BACKGROUND OF THE INVENTION

The AFM is a well-known device in which the topography of a sample ismodified or sensed by a probe or probe mounted on the end of amicrofabricated cantilever. As the sample is scanned, the interaction ofatomic forces between the probe and the sample surface causes pivotaldeflection of the cantilever. The topography of the sample may bedetermined by detecting this deflection of the probe. By controlling thedeflection of the cantilever and the physical properties of the probethe surface topography may be modified to produce a pattern on thesample.

Lithography is the process for producing a pattern of two dimensionalshapes made by drawing primitives such as lines and pixels within alayer of material, such as, for example, a resist coated on asemiconductor device. Conventional photolithography is running intosevere problems as the feature size is reduced below 65 nm. Theseproblems arise from fundamental issues such as sources for the lowwavelength of light, photoacid migration, photoresist collapse, lenssystem quality for low wavelength light and masks cost. To overcomethese issues an alternative approach to the generation of sub-50 nmfeatures is required.

One approach to addressing these issues is to use a scanning probe-basedlithography (SPL) device. In this strategy a probe is raster scannedacross a resist surface and brought to locally interact with the resistmaterial. By this interaction resist material is removed or changed in away that it can be developed.

SPL is described in detail in Chemical Reviews, 1997, Volume 97 pages1195 to 1230, “Nanometer-scale Surface Modification Using the ScanningProbe microscope: Progress since 1991”, Nyffenegger et al. and thereferences cited therein. In particular, the article describes use oforganic materials as lithographically active layers (see pages 1219 and1220) which have been used in mechanical modicon experiments to generateline patterns in the organic resist material.

In the mechanical modification experiments the drawing primitives arephysically realized as indentations in the polymer layer created byheating the cantilever probe and with the application of force pressingthis probe into the polymer. The probe is heated by passing a currentthrough a resistive heater integrated in the cantilever, directly behindthe probe. Some of the heat generated in the resistor is conductedthrough the probe and into the polymer layer, locally heating a smallvolume of the polymer. If sufficient heat is transferred to raise thetemperature of the polymer above a certain temperature (which isdependent on the chosen polymer), the polymer softens and the probesinks in, creating an indentation or line. Examples of organic resistmaterials used were poly(methyl methacrylate) (PMMA) and poly(glycidylmethacrylate) PGMA.

There are a number of problems with the known implementations of SPL.These problems include difficulty with crossing lines if material is notproperly removed but only squashed to the side by the interacting probe,contamination of the probe if material is not removed, and if a layer isstructured by indenting or scratching with the probe, the probe tends towear.

The efficiency of forming indentations is therefore critically dependenton the nature of the polymeric thin film used as the resist. Desirableattributes of the polymeric thin film are ‘softness’ and deformabilityduring the writing phase, stability, toughness and resistance to wearduring mechanical and chemical processing. A hard polymer with a highmelting point will be difficult to soften sufficiently for the probe tosink in and form the pit during the writing process. Linear polymerssuch as PMMA have been found to have suitable writing temperatures andthe force required on the probe to form the indentation is acceptablylow for the required probe performance and power consumption; however,rim formation around the indentations and the erasing of indentationswhen writing indentations in close proximity is a problem. Such problemswould be overcome by using a harder crosslinked polymer but this wouldlead to unacceptable probe wear rate during writing.

An example of scanning thermal lithography using polymers which undergoselective crosslinking is disclosed athttp://www.eecs.umich.edu/˜basualpages/SThL. html.

The present invention seeks to overcome these problems by using a classof polymers which under controlled conditions have the characteristicsof linear polymers and are thus suitable for the writing phase but havethe characteristics of crosslinked polymers during subsequent processingsteps.

SUMMARY OF THE INVENTION

The invention provides a resist medium in which features arelithographically produced by scanning a surface of the medium with anAFM probe positioned in contact therewith, said medium comprising asubstrate; and a polymer resist layer within which features are producedby mechanical action of the probe, characterized in that the polymercontains thermally reversible crosslinkages.

Wear of the AFM probe is minimized using the invention becausemechanical contact of the probe with the resist during the lithographicprocess is with polymer having crosslinkages broken or opened by theheating effect of the probe; the softer polymer being easier tomechanically pattern than the fully crosslinked polymer. The polymersare preferably non-adhesive in the uncrosslinked state to avoidcontamination of the probe.

Reference to a heated probe is used herein to refer to a combination ofheat and force being used. The present invention also provides alithography system comprising an AFM head having a probe connected to aresistive path locally exerting heat at the probe when an electricalcurrent is applied; and a resist medium comprising a polymer containingthermally reversible crosslinkages as hereinbefore described.

The lithography system preferably includes a plurality of AFM headsarranged in the form of at least one array enabling a parallel approachto producing the pattern on the resist medium and thereby increasing thepatterning speed. More preferably the resist medium is arranged to bemoved in a predetermined direction and the AFM head array is fixedthereby enabling the resist medium to be scanned by the head array.Alternatively the AFM head array may be moveable and may be scannedacross a fixed resist medium.

In yet a further embodiment of the present invention there is provided alithographic process for producing features in a resist mediumcomprising arranging a heat emitting probe connected to a resistivepath, in contact with a resist medium as hereinbefore described; drivinga current through the resistive path thereby heating the probe to apredetermined feature forming temperature thereby causing localsoftening of the organic polymer resist layer allowing the AFM probe topenetrate the resist layer to form an indentation in the resist.

In a preferred embodiment the heat and force applied to the probe arevaried to control the penetration of the probe into the resist mediumenabling indent height to be varied; this approach enables 3-dimensionalpatterning producing multilevel structures within the resist layer. Thisis particularly useful if the topography of the surface beneath theresist varies because of the presence of features on the substrate.

The present invention may advantageously used in prototyping patternedresists. Unlike conventional production of patterned resists, thepresent invention enables a suitable software controlled AFM head orarray of heads to “write” the pattern on the resist. In effect theconventional mask used in optical lithography may be produced in situ bythe process of the present invention.

The present invention further provides a patterned resist medium whichmay be scanned with an AFM probe positioned in contact therewith to readthe pattern in order to check whether the desired patterning has beenformed in the lithographic process.

As described above, the polymer resist of the present invention containsthermally reversible crosslinkages. Crosslinking between polymer chainscan be effected in a number of ways. Crosslinks may be achieved byhaving pendant groups on the polymer chains which may be linked directlyor via linking agents. The crosslinks of the present invention must becapable of being severed upon heating and preferably may reformspontaneously on cooling.

The polymer resist medium is most suitably an organic polymer. Thethermally reversible crosslinkages are preferably based on covalentbonds. A particularly preferred polymer resist is an organic polymercomprising polymer chains which are connected one to another withDiels-Alder adducts. As used herein reaction is a cycloadditionreaction, also referred to as a conjugate reaction, in which an alkene,also referred to as a dienophile, adds to a 1,3-diene, that is, aconjugated diene, to form a six-membered ring

A Diels-Alder adduct of formula (Structure I in the Appendix): whereinX1 and X2 may be the same or different and are electron attractingsubstituents, R1 is hydrogen, R2 is hydrogen, Y1 and Y2 may be the sameor different and are hydrogen, alkyl or substituted alkyl, is formed bythe reaction of a diene of formula (Structure II in the Appendix)wherein X1 and X2 are as hereinbefore defined, and a dienophile offormula (Structure III in the Appendix) wherein R1, R2, Y1 and Y2 are ashereinbefore defined. The diene and dienophile, and the adduct producedtherefrom are suitably attached directly or indirectly to the polymerbackbone by at least one of X1 and X2, and R1, R2, Y1 and Y2respectively.

The Diels-Alder reaction is reversible and the rate of reaction betweenthe diene and the dienophile to form the adduct, and the rate of thereverse reaction may be controlled by appropriate selection of the dieneand dienophile and the substituents X1 and X2, and R1, R2, Y1 and Y2attached thereto.

Crosslinked resins comprising polymer chains connected to one another byDiels-Alder adducts are known from, for example, U.S. Pat. No. 5,641,856in the name of Shell Oil Company and U.S. Pat. No. 6,271,335 in the nameof Sandia Corporation.

One advantage of using thermally reversible crosslinked materials, suchas for example those based on Diels-Alder chemistry is that because thecrosslinks are thermally reversible, as a result the material undergoesa dramatic change from a tough crosslinked solid to a soft, viscous meltof lower molecular weight fragments as the temperature is raised abovethe critical temperature at which the reverse Diels-Alder reactionoccurs.

The “writing” mechanism of these polymers appears to be different fromthat observed for the polymers of the prior art. The polymers of theprior art are patterned using a ‘squashing-type’ mechanism wherematerial is compacted but no significant molecular diffusion occurs;large rims around the indentations are formed. The much lower viscosityof the thermally reversed, non-crosslinked form of the polymers of thepresent invention leads to significant diffusion of the molecules duringwriting and as a result the rim around the indentation does not form tothe same extent; the overall dimension of the indentation issignificantly decreased. An overall decrease in indentation dimensionleads to increased density of patterning because the indentations can bepacked closer together.

One particular class of Diels-Alder crosslinked polymers suitable foruse in the present invention is the group of polymers having Diels-Alderadducts formed from a dienophile and a substituted furan.

Suitable furans include those of formula (Structure IV in the Appendix)wherein R3 represents hydrogen or an alkyl group and R4 represents afunctional group linking to a polymer chain. Preferably R3 representshydrogen or a methyl group. Preferred furans within formula (StructureIV in the Appendix) are the polymeric materials of formula (Structure Vin the Appendix) wherein n and m represent the number of oligomericunits in the polymer chain.

Preferred dienophiles for use in the present invention are derivativesof maleimide. Suitable maleimides include those of formula (Structure VIin the Appendix) wherein R5 represents a functional group linking to apolymer chain. Preferred maleimides within formula (Structure VI in theAppendix) are the polymeric materials of formula (Structure VII in theAppendix) wherein n and m represent the number of oligomeric units inthe polymer chain.

The Diels-Alder adduct formed by the reaction of the furan of formula(Structure V in the Appendix) and the maleimide of formula (StructureVII in the Appendix) may be represented by formula (Structure VIII inthe Appendix) wherein n and m are as defined above. The Diels-Alderadduct of formula (Structure VIII in the Appendix) is a tough highlycrosslinked polymer which cleaves to form the furan (Structure V in theAppendix) and maleimide (Structure VI in the Appendix) at temperaturesgreater than 140° C. The mixture of furan (Structure V in the Appendix)and maleimide (Structure VI in the Appendix) is a soft material withviscous fluid properties. At temperatures below about 130° C. theDiels-Alder adducts reform to produce the tough highly crosslinkedpolymer. For use in the lithography media of the present invention thepolymer of formula (Structure VII in the Appendix) would require awrite-temperature in excess of 140° C.

In an alternative embodiment the polymer is a silicone derivative. Thefuran of formula (Structure IX in the Appendix) wherein R6 is alkyl orcycloalkyl, may be reacted with a maleimide of formula (Structure X inthe Appendix) wherein R7 represents —CH₂—, —CH₂CH₂—, or 1,4-phenylene toproduce a crosslinked network polymer. The degree of crosslinking may becontrolled by adding furan to the reaction mixture as a chain lengthinhibitor. Preferably R6 is cyclohexyl. Preferably R7 is 1,4-phenylene.

The crosslinked network may be represented by formula (Structure X1 inthe Appendix) wherein R6 and R7 are as hereinbefore defined.

The properties of the crosslinked polymeric material are chosen suchthat the material can be spin-cast onto the substrate to give a uniformthickness of film at the desired thickness. One suitable method of spincoating requires the polymeric material to be spin coated onto thesubstrate in an uncrosslinked state

-   -   this may be done by using a temperature above which the cross        links are broken.

In an alternative preferred method, the spin coating may be done at alower temperature using a diene and/or dienophile precursor which isconverted to the diene and/or dienophile after the spin coating.Suitable precursors of the dienophile include protected dienophiles. Amultifunctional diene as described above and a protected multifunctionaldienophile are mixed and spin coated onto the substrate to give a thinpolymer film of the desired thickness. It is preferable to use aprotected, multifunctional dienophile since this allows a stable mixtureof the two components to be prepared which in turn allows thin films ofreproducible thickness to be obtained. Heating of the thin film ofpolymer then leads to deprotection of the dienophile which undergoesreaction with the multifunctional diene via Diels Alder chemistry togive a highly crosslinked thin film. Such a highly crosslinked film istough and resistant to wear at temperatures below the reverse DielsAlder reaction temperature; however, above this critical temperature thereverse Diels Alder reaction occurs to break the crosslinks and theoriginal precursor molecules are obtained. Since these originalprecursor molecules are lower molecular weight, non-crosslinkedmaterials, the thin films becomes very soft and writing is much easierthan for a similar thin film composed on non-reversible crosslinks. Themultifunctional diene and dienophile may be small organic molecules,they may also be functionalized linear chains, branched polymers, blockcopolymers, dendrimers, hyperbranched macromolecules or mixturesthereof.

The transition temperature between the crosslinked state and theuncrosslinked state, which may be written as Tr, may also be referred toas the crosslinkage cleavage temperature. The transition temperaturebetween the crosslinked and the uncrosslinked material described abovemay be readily determined for any crosslinked polymer byexperimentation. For example, the transition temperature for the polymerof formula (Structure VII in the Appendix) is between about 130° C. and140° C., the transition temperature for the polymer of formula(Structure X1 in the Appendix) wherein R6 is cycloalkyl. and R7 is1,4-phenylene is about 120° C.

The class of crosslinked polymers described above which are connectedone to another with Diels-Alder adducts is one example of suitablepolymeric materials for use in the present invention. Any class ofcrosslinked polymer in which the crosslinks may be cleaved thermally aresuitable for use in the present invention subject to the proviso thatthe cleavage temperature is within the working temperature range of theSTM probe. The crosslinked polymers described above contain covalentthermally reversible crosslinks. In an alternative embodiment thecrosslinkages are suitably non-covalent bonds.

In a preferred alternative embodiment the crosslinkages are hydrogenbonds (H-bonding). One advantage for the use of non-covalentcrosslinkages such as hydrogen bonds is the potential to tailor thestrength of the crosslinking by changing the number and nature of thenon-covalent interactions. If H-bonding is used, crosslinks may involveindividual H-bonds or quadruple H-bonds which have a significantlyhigher dissociation temperature than the materials formed fromindividual H-bonds thereby modifying the operating parameters of thelithography medium One suitable example of a quadruple H-bonding systemis shown in Reaction Scheme 1.

The crosslinked polymer of formula (Structure XII in the Appendix),wherein P is the polymer backbone and R is hydrogen or an alkyl group issuitably formed by dissolving a linear polymer of formula (StructureXIII in the Appendix) containing the H-bonding crosslinking units in apolar solvent which is chosen for its H-bonding character and ability todisrupt the H-bonding between the crosslinking units. This solution isthen spin-cast onto an appropriate substrate such as those describedabove to give a thin medium of the desired thickness. The solvent isevaporated and as the solvent is removed the H-bonding units along thepolymeric backbone start to form crosslinks leading to formation of acrosslinked polymer (Structure XII in the Appendix).

The highly crosslinked polymer (Structure XII in the Appendix) is toughand resistant to wear at temperatures below the temperature at which theH-bonds break.

For quadruple H-bonding units the temperature at which the crosslinksare cleaved is about 80 to 100° C. Above this critical temperature theH-bonding units break removing the crosslinks and the original precursormolecules are obtained.

The precursor molecules are lower molecular weight, non-crosslinkedmaterials, and as for the covalent thermally reversible crosslinkedmaterials described above, the thin film becomes very soft andpatterning is much easier than for a similar thin film composed onnon-reversible crosslinks. The H-bonding units may be small organicmolecules, functionalized linear chains, branched polymers, blockcopolymers, dendrimers, hyperbranched macromolecules or mixturesthereof.

The thickness of the polymer resist layer is suitably in the range 2 to1000 nm, more suitably 2 to 200 nm, most suitably 2 to 50 nm, with athickness of about 5 nm being preferred.

The substrate upon which the polymer resist layer is deposited is mostsuitably silicon. The silicon substrate may be patterned or modifieddepending upon the stage in the fabrication process the presentlithographic process is being used. Other suitable substrates for use inthe present invention may be electrically conducting or non-conducting,and include metallic surfaces and conventional insulators such as, forexample, silicon dioxide.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexample, with reference to the accompanying drawings in which:

FIGS. 1 a to 1 c illustrate the construction and operation of apatterning/read component for a lithographic device including the resistmedium of the present invention;

FIG. 2 is a schematic representation of a lithographic device in whichthe resist medium of the invention can be employed.

FIGS. 3 a to 3 f are a schematic partial cross sectional representationof a lithographic process using a resist medium of the presentinvention.

FIG. 4 is a Scanning Force Microscopy image of a resist medium of thepresent invention showing a pattern written in the resist using thelithographic process of the present invention.

FIG. 5 is a Scanning Force Microscopy image of a resist medium of thepresent invention and a conventional resist medium showing exposure ofan area with varying exposure temperature.

FIG. 6 is graph showing the line-depth versus probe-temperature diagramfor the resist medium of the present invention shown in FIG. 5.

FIG. 7 is a graph showing the “writing” kinetics for a resist of thepresent invention and a conventional resist.

DESCRIPTION OF THE INVENTION

A patterning/read component 1 of an AFM-based lithographic device isshown schematically in FIGS. 1 a to 1 c of the accompanying drawings.The component 1 comprises a generally U-shaped cantilever 2 which isconnected to a support structure 3 (only partially shown in thefigures). Flexing of the legs 2 a, 2 b of the cantilever 2 provides forsubstantially pivotal movement of the cantilever about a pivot axis P.The probe 4 is provided on a heater 5 which forms a platform at the endof the cantilever 2. The highly-doped silicon cantilever legs 2 a, 2 bdefine a current path connecting the heater platform 5 between a pair ofelectrical supply lines (not shown) on the support structure 3.

In operation, the probe 4 is biased against the surface of the resistmedium of the present invention indicated schematically at 6 and shownhere in cross-section.

The resist comprises a substrate 6 a and a resist layer 6 b.

In the patterning mode, the heater platform 5 can be heated to apatterning temperature TW by application of a patterning-mode potentialacross the supply lines. The consequent heating of the probe 4 resultsin heat transfer to the resist layer 6 a causing local softening of thepolymer. This allows the probe 4 to penetrate the surface layer to forma pit, or indentation, 7 as shown in FIG. 1 a. The resist medium 6 canbe moved relative to patterning/read component 1 allowing the probe towrite patterning over an area of the surface corresponding to the fieldof movement.

Another important aspect of the present invention is the fact that thesuccess of the writing can be checked in situ by means of topographicimaging of the indentations created in the writing process. The processof reading back the topography from the lithography medium involvessensing the height of the probe with respect to a predeterminedreference level. This may be accomplished by means of thermo-mechanicalsensing which entails mechanically connecting a heat emitting resistivepath to the probe, driving a current through the resistive path to apredetermined temperature and determining the thermal conductancebetween the resistive path and the storage medium. The thermalconductance between the resistive path and the resist medium is afunction of the distance between the heat emitting path and the surfaceof the resist medium. Alternatively, any other method for sensing theheight of the probe known in the art of local probe microscopy can beemployed for imaging purposes. Prominent examples are optical deflectionsensing, optical interferrometric sensing, piezo resistive sensing,piezo electric sensing.

In the illustrated example of thermo-mechanical sensing, in the “read”mode, the heater platform 5 is used as a thermal sensor by exploitingits temperature-dependent resistance. A read-mode potential is appliedacross the supply lines to heat the heater to a reading temperature TRwhich is less than the writing temperature TW and not high enough tocause softening of the polymer. As the resist surface is scanned by theprobe 4, the pivotal position of the cantilever 2 at each indentationposition differs according to the presence or absence of an indentation7. In the absence of an indentation, as shown in FIG. 1 b, the distancebetween the heater platform 5 and storage medium 6 is greater than thecorresponding distance when an indentation is present, and the probeenters the indentation, as shown in FIG. 1 c. Heat transport across theair gap between the heater 5 and resist medium 6 is thus more efficientwhen an indentation is present, and since more heat is then lost to theresist medium, the temperature of the heater 5, and hence itsresistance, will be reduced. Thus, as the resist surface is scanned, theposition of indentations is detected by monitoring changes in thetemperature of the heater 5, in practice by monitoring changes in thevoltage across a series resistor in one of the supply lines.

FIG. 2 depicts a lithographic device 20 including a resist medium 21 anda lithographic apparatus as described above in the form of an array 22of patterning/read components 23. It is to be appreciated, however, thatthese components may include additional circuitry, such as amplifiersetc., where required. Each patterning/read component 23 is connected totwo supply lines, a row supply line R and a column supply line C, asindicated schematically in the FIG. 2. All components 23 in the same rowof the array share the same row supply line R. Similarly, all componentsin the same column of the array share the same column supply line C.Drive means, indicated schematically at 24, enables the relativemovement of the array and resist medium, whereby the array can beaccurately located in its operating position against the resist medium.

The row and column lines R, C of array 22 are connected to power supplyand read detection circuitry indicated generally at 25. Circuitry 25operates to supply power to the components of the array 22, theindividual components 23 being addressed by their row and column linesin known manner via row and column multiplexers (not shown) of circuitry25. Each component 23 can be addressed in both a patterning mode and aread mode, the power supply circuitry supplying a “write” signal via thesupply lines in the patterning mode, and a read mode signal via thesupply lines in the read mode. The resist medium 21 comprises asubstrate having a thin layer of the polymer of the present inventionspin coated thereon. The pattern is written in and read from the resistmedium as described in relation to FIG. 1 above.

FIGS. 3 a to 3 f are a schematic partial cross sectional representationof a lithographic process using a resist medium of the presentinvention. FIG. 3 depicts an intermediate stage in the production of asemiconductor device. The partially constructed semiconductor devicecomprises a substrate 100 on which a number of features 101, 102 havebeen produced in an area for the construction of devices 120. Inaddition the substrate carries a mask alignment area 103 including analignment mark 104 from a previously formed layer. A resist layer 105 ofthe present invention has been deposited on the upper surface of thesemiconductor device using known resist deposition techniques. Thecantilever 106 and the probe of the AFM probe are shown above thesurface of the resist.

In FIG. 3 b the resist layer 105 in the region of the mask alignmentarea 103 has been removed to reveal the alignment mark 104. The removalof the resist layer 105 is preferably carried out using the lithographicprocess of the present invention.

As illustrated in FIG. 3 c, prior to the lithographic process being usedto produce further indentations in the resist, it is necessary for theprobe to be accurately aligned with the partially constructedsemiconductor device. Alignment is effected by reading the topography atthe mask alignment area 103 and moving the probe 107 or substrate 100 toalign the probe 107 with the alignment mark 104.

Under the control of suitable probe controlling software, the probe 107is moved across the resist layer 105 and heat applied at predeterminedlocations 108, 109, 110 to cause the probe to displace(expose/evaporate) the resist layer to produce indentations exposing theupper surface of the substrate 100. FIG. 3 d shows in partial crosssection the desired layout of indentations and exposed substrate. Theresult of a substrate removal process is shown in FIG. 3 e wheretrenches 111, 112, and 113 are shown in the substrate 100 having beenformed by a removal process, such as for example etching, in the regions108, 109, and 109 produced by the lithographic process of the presentinvention.

FIG. 3 f shows the substrate 100 after removal of the remaining resistlayer. The substrate is available for further processing and the stepsshown in FIGS. 3 a to 3 f may be repeated to produce a furtherlithographically produced pattern.

FIG. 4 is an Atomic Force Microscope image of a resist medium of thepresent invention. The indentations appear as dark areas on theotherwise gray background which is the polymer resist surface. Thepolymer and polymer film used to obtain the image were prepared asdescribed in the following examples.

FIG. 5 is a Scanning Force Microscopy image of a resist medium of thepresent invention and a conventional resist medium showing exposure ofan area with varying exposure temperature. The exposed area in FIG. 5consists of 2.5 micrometer long lines closely spaced over 5 micrometers.Each line has been exposed for the same time but with varying exposuretemperature. From this the cross-section in FIG. 6 was extractedallowing estimation of the exposure depth as a function of temperaturefor the given tip and scan speed. It becomes apparent that a thresholdtemperature is needed to remove material (i.e. during exposure), belowthat threshold the resist remains unchanged and no resist medium wearcan be observed. For comparison uncrosslinked materials such as, forexample, PMMA will show indentations in the resist medium.

FIG. 6 is graph showing the line-depth versus probe-temperature diagramfor the resist medium of the present invention shown in FIG. 5. Thepolymer used is a mixture of compound of Formula IX and of Formula Xwherein R7 is 1,4-phenylene, prepared as hereinbefore described usingthe furan protection method described.

FIG. 7 is a graph showing the “writing” kinetics of a single pixel for aresist of the present invention (thin solid line/circles) and aconventional resist (thick solid line/filled squares). The conventionalresist used is poly methyl methacrylate. The resist of the presentinvention is as described above in relation to FIG. 5. It can clearlyseen that exposure times of 10 microsecond per pixel are readilyfeasible requiring a heater temperature of approximately 500° C. Evenshorter exposure times below 1 microsecond or even 100 nanoseconds arereadily feasible for a resist of the present invention as the requiredwrite temperatures would only be a few tens of degrees higher. Forconventional polymer resists, however, prohibitively high writingtemperatures in excess of 700° C. would be required in order to achievethe required exposure for writing times below 1 microsecond.

The following examples illustrate the preparation and use of organicpolymers and their precursors for use in the recording surfaces of thepresent invention:

General Methods: Commercial reagents were obtained from Aldrich and usedwithout further purification. Analytical TLC was performed on commercialMerck plates coated with silica gel GF254 (0.24 mm thick). Silica gelfor flash chromatography was Merck Kieselgel 60 (230-400 mesh, ASTM).Nuclear magnetic resonance was performed on a Bruker AVANCE 400 FT-NMRspectrometer using deuterated solvents and the solvent peak as areference. Gel permeation chromatography was performed intetrahydrofuran (THF) on a Waters chromatograph equipped with four 5-mmWaters columns (300×7.7 mm) connected in series with increasing poresize (100, 1000, 100,000, 1,000,000 Å). A Waters 410 differentialrefractometer and a 996 photodiode array detector were employed. Thepolystyrene molecular weights were calculated relative to linearpolystyrene standards, whereas the poly(n-butyl acrylate) molecularweights were calculated relative to poly(n-butyl acrylate) standards.

EXAMPLE 1

Cross-link mechanism by hydrogen bonding: Multihydrogen BondingPolystyrene (MHB-PS). (reference Advanced Materials 2000, 12, 878).

To a solution of PS-VBA 1 (1.5 g, 0.3 mmol) in chloroform (30 mL) wasadded 2-(6-isocyanatohexylaminocarbonylamino)-6-methyl-4{1H}pyrimidinone(1 g, 3.2 mmol). After addition of catalytic amount of dibutyltindilaurate, the resulting solution was stirred, refluxed overnight. Aftercooling down, chloroform (50 mL) was added and the solution wasfiltered. After concentrating the solution back to 30 mL, 1 g of silicaand 1 drop of dibutyltin dilaurate were added, and the mixture wasrefluxed for additional 1 h. The silica was removed by filtration andthe chloroform was partially removed. The viscous mixture wasprecipitated in methanol. The white powder was then collected and driedto give the desired copolymer (1.19 g). 1H NMR (400 MHz, CDCl3) 13.04(br s, 1H, intramol H bond), 11.8 (s, 1 H, CH2NHCONH), 10.07 (s, 1H,CH2NHCONH), 7.44-6.35 (m, 57H), 5.74 (s, 1H, CH═CCH3), 4.94 (s, 2.3H,CH2OCONH), 4.50 (s, 4.1 H, CH2OH), 3.18-3.10 (m, 4H, CH2NCO andCH2NHCONH), 2.20-0.42 (m, 53.5H); IR (neat) 3419, 336, 3220, 3058, 3025,2925, 2854, 1700, 1662, 1585, 1521, 1493,1452, 1251, 1029, 817, 761, 700cm-1; Mn=5919, PDI is 1.05. Tg=122.8° C.

EXAMPLE 2

2-Methyl-acrylic acid2-[3-(6-methyl-4-oxo-1,4-dihydro-pyrimidin-2-yl)-ureido]-ethyl ester.(MH B-MA) (reference Journal of the American Chemical Society 1998, 120,6761)

A suspension of 6-methylisocytosine (0.98 g, 7.8 mmol), and2-isocyanatoethyl methacrylate (2.20 g, 14.1 mmol) in dry pyridine (35mL) was heated under reflux for 2 h, giving a clear solution. Coolinginduced the formation of crystals. Acetone was added (20 mL), and theresulting microcrystalline powder was filtered. Recrystallization fromethanol/CHCl3 (1:1, v/v) gave analytically pure product (1.62 g, yieldis 74%) 1H NMR (400 MHz, CDCl3) 13.00 (s, 1H, intramol H-bonding), 11.97(s, 1H, CH2—NH—CO), 10.53 (s, 1H, CH2—NH—CO—NH), 6.20 (s, 1H, CHH═C),5.80 (s, CCH3CH—CO), 5.56 (s, 1H, CHH═C), 4.31-4.28 (t, J=5.6, 2H,OCH2CH2N), 3.62-3.58 (q, J=5.6 Hz, 2H, OCH2CH2N), 2.26 (s, 3H,CH3—C—NH), 1.96 (s, 3H, CH3—C═CH2), IR (neat) 3250-2800, 1726, 1699,1664, 1641, 1583, 1521, 1253, 1172, 939 cm-1; Melting point Tm=206.7° C.

EXAMPLE 3

Synthesis of Tris(furfuryloxy)cyclohexylsilane

To a 1000 ml flask was added 100 ml dry of toluene, furfuryl alcohol(70.00 g, 718 mmol) and trethylamine (75.35 g, 746 mmol) was addeddropwise a solution of cyclohexyltrichlorosilane (49.2 g, 226 mmol) indry toluene (150 ml). The reaction mixture was then stirred overnight atroom temperature and the heavy suspension was then stirred with diethylether (500 ml) and filtered. The precipitate was then triturated withdiethyl ether (500 ml) and the combined organic layers dried andevaporated to dryness. The crude product was purified by distillation(169-173 C at 500 mTorr) to afford the tris(furan) as a light yellow oil(83.9 g, 92%). 1H-NMR (400 MHz, CDCl3) d: 7.40 (m, 3H, ArH), 6.33 (m,3H, ArH), 6.24 (m, 3H, ArH), 4.72 (s, 6H, OCH2), 1.82-1.88 (m, 4H, CH2),1.65-1.70 (m, 6H, CH2), and 0.85 (t, 1H, CH); 13C-NMR (100 MHz, CDCl3)d: 153.5, 142.4, 110.0, 107.8, 57.4, 27.6, 26.6, 26.4, 22.9.

EXAMPLE 4

Bis Furan protected derivative of 1,1(Methylenedi-4,1-phenylene)bismaleimide

The bismaleimide (45.45 g, 127 mmol) was dissolved in tetrahydrofuran(100 ml) and furan (51.80 g, 762 mmol) was added dropwise. The reactionmixture was then heated at reflux under nitrogen for 6 hours and thenleft to stir at room temperature overnight followed by evaporation todryness. The crude product was purified by filtration through silica togive the bis(furan) protected derivative as a white solid (55.0 g, 88%);1H-NMR (400 MHz, CDCl3) d: 7.12 and 7.26 (ABq, 8H, J=7.4 Hz, ArH), 6.49(s, 4H, Alkene-CH), 5.50 (s, 4H, CH—O), 3.96 (s, 2H, CH2), and 2.94 (s,4H, CH).

EXAMPLE 5

Bis 2-Methylfuran protected derivative of7,7-dihexyl-1,14-(bismaleimide)tetradecane.

The bismaleimide (Loctite) (5.00 g, 7.9 mmol) was dissolved intetrahydrofuran (30 ml) and 2-methylfuran (1.30 g, 15.8 mmol) was addeddropwise. The reaction mixture was then heated at reflux under nitrogenfor 18 hours and evaporated to dryness. The crude product was purifiedby filtration through silica to give the bis(2-methylfuran) protectedderivative as a clear oil (4.50 g, 70%); 1H-NMR (400 MHz, CDCl3) d: 6.58(d, 2H, J=1.4 Hz, Alkene CH), 6.58 (d, 2H, J=1.1 Hz, Alkene CH), 5.05(s, 2H, CH—O), 3.43 (t, 4H, N—CH2), 2.82 and 3.01 (each d, 2H, CH), 1.72(s, 6H, furan-CH3), 1.30-1.65 (m, 36H, CH2), and 0.90 (t, 6H, CH3).

EXAMPLE 6

Formulation of Prepolymer (B-staging via in-situ protection)

1,1-(Methylenedi-4,1-phenylene)bismaleimide (6.65 g, 18.6 mmol) wasdissolved in NMP (20 ml); tris(furfuryloxy)cyclohexylsilane of Example 3(5.00 g, 12.4 mmol) and furan (1.68 g, 24.8 mmol) were then added andthe reaction mixture stirred at room temperature for 48 hours. Excessfuran (8.41 g, 124 mmol) was then added followed by stirring at roomtemperature for 24 hours to protect all of the remaining maleimidegroups. The reaction mixture was then precipitated (2×) into hexane (500ml) to give the B-staged Diels Alder polymer (11.5 g, 86%). Themolecular weight, viscosity and spinning characteristics of the B-stagedpolymer could be controlled by the ratio ofbis(maleimide):tris(furan):furan in the original reaction mixture.1H-NMR (400 MHz, CDCl3) d: 7.54-6.30 (m, ArH and alkene CH), 4.60-3.00(br m, CH2 and CH), and 1.05-2.20 (m, CH2).

EXAMPLE 7

Formulation of Prepolymer (B-staging via protected monomers).

A mixture of the bis(furan) protected bismaleiimide of Example 4 (9.34g, 18.6 mmol) and the tris(furfuryloxy)cyclohexylsilane of Example 3(5.00 g, 12.4 mmol) were dissolved in NMP (20 ml) and heated at 80 C for12 hours. An excess of furan (8.41 g, 124 mmol) was then added followedby stirring at room temperature for 24 hours to protect all of theremaining deprotected maleimide groups. The reaction mixture was thenprecipitated (2×) into hexane (500 ml) to give the B-staged Diels Alderpolymer, 5, (12.1 g, 90.5%). The molecular weight, viscosity andspinning characteristics of the B-staged polymer prepared by thisalternate method could be controlled by the ratio ofbis(maleimide):tris(furan):furan in the original reaction mixture, thereaction temperature and reaction time. 1H-NMR (400 MHz, CDCl3) d:7.54-6.30 (m, ArH and alkene CH), 4.60-3.00 (br m, CH2 and CH), and1.05-2.20 (m, CH2).

EXAMPLE 8

Fabrication of Thin Crosslinked Films.

The B-staged polymer (1.0 g) was dissolved in dry NMP (9.0 g) (10.0 wt %solution) and this solution was then filtered through a 0.1 mm filterand spun coated onto silicon wafers at 2500 rpm. The wafers were thenheated at 120 C for 5 hours to cause full deprotection of the maleiimidegroups with associated loss of the furan protecting group andcrosslinking. The resulting thin films, the thickness of which could becontrolled by spin speed and initial wt %, were shown to be fullycrosslinked and defect free, suitable for use as a resist polymer in thelithographic process of the present invention.

While the invention has been described with respect to certain preferredembodiments and exemplifications, it is not intended to limit the scopeof the invention thereby, but solely by the claims appended hereto.

1. A method, comprising: providing a substrate over which is located alayer of polymer resist, wherein the polymer resist contains thermallyreversible crosslinkages; and scanning a surface of the polymer resistlayer with an AFM probe positioned in contact with the resist layer, toproduce features in the layer by mechanical action of the probe.
 2. Amethod as claimed in claim 1, wherein the polymer resist is an organicpolymer.
 3. A method as claimed in claim 2, wherein the thermallyreversible crosslinkages are covalent bonds.
 4. A method as claimed inclaim 3, wherein the thermally reversible crosslinkages are Diels-Alderadducts.
 5. A method as claimed in claim 3 wherein the Diels-Alderadduct is the product of a dienophile and a substituted furan.
 6. Amethod as claimed in claim 1, wherein the thermally reversiblecrosslinkages are non-covalent bonds.
 7. A method as claimed in claim 6,wherein the non-covalent thermally reversible crosslinkages are hydrogenbonds.
 8. A method as claimed in claim 1, wherein the polymer resist isa silicone derivative.
 9. A lithography system, comprising: an AFM headhaving a probe adapted to be heated; and a resist medium comprising asubstrate and a polymer resist layer within which features are producedby mechanical action of the probe, wherein the polymer resist containsthermally reversible crosslinkages.
 10. A lithography system as claimedin claim g, comprising an AFM head having a probe connected to aresistive path for locally exerting heat at the probe when an electricalcurrent is applied.
 11. A lithography system as claimed in claim 10,wherein a plurality of AFM heads are arranged in the form of at leastone array.
 12. A lithography system as claimed in claim 11, wherein theresist medium is arranged to be rotated and the AFM head array is fixedand has a form adapted to circular geometry.
 13. A process for producingfeatures in a resist medium, comprising: arranging a heat-emitting probethat is connected to a resistive path, so that the probe is in contactwith a resist medium that includes a substrate and a polymer resistlayer within which features are produced by mechanical action of theprobe, wherein the polymer resist contains thermally reversiblecrosslinkages; and driving a current through the resistive path to heatthe probe to a feature-forming temperature, thereby causing localsoftening of the organic polymer resist layer and allowing the AFM probeto penetrate the resist layer to form an indentation.
 14. A process asclaimed in claim 13 for producing a pattern of two-dimensional shapesmade by drawing primitives selected from lines and pixels.
 15. A processas claimed in claim 13, wherein the probe lithographically removes anarea of the resist medium to expose alignment marks on the substrate,thereby permitting the probe and the substrate to be aligned prior tofurther lithographic processing.
 16. A process for reading thetopography of a resist medium patterned by a process as claimed in claim13, wherein the resistive path is mechanically coupled to a tip placedin contact with the resist medium, comprising: driving a current throughthe resistive path and determining the thermal conductance between themedium and the resistive path.
 17. A process for reading the topographyof a resist medium patterned by a process as claimed in claim 13,comprising: placing a tip in contact with the resist medium; andoperating a position sensor for measuring a vertical position of the tipwith respect to a reference plane in the resist medium.
 18. A processfor use in the production of a semiconductor device, comprising:depositing on the upper surface of a semiconductor substrate a polymerresist layer containing thermally reversible cross linkages and withinwhich features are produced by mechanical action of an AFM probepositioned in contact therewith; and moving the probe across the resistlayer and applying heat at predetermined locations to cause the probe todisplace the resist layer to produce indentations exposing the uppersurface of the substrate.